It is known from WO 98/19869 to incorporate a security feature in the form of a perforation pattern into documents such as banknotes, passes, or plastic cards, for example payment cards. In that case a plurality of perforations are applied extensively in side-by-side relationship.
In that respect the attempt is made to produce an extensive two-dimensional image by control of the diameters of the perforations, that is to say the mouth openings of the perforations on one of the outward sides, or by way of the spacings between the mouth openings, that is to say the distribution of the mouth openings on the surface. In accordance with WO 00/43216 that is possible even when the arrangement involves blind holes which do not at all reach the viewing side.
In practice that method is implemented by a procedure whereby perforations are burnt in by means of laser light in mutually juxtaposed relationship from the rear side of the document, that is to say the substrate, in regular or irregular raster configurations. Accordingly the mouth openings of the perforations on that rear side are all of the same size, but not the mouth openings of the corresponding perforations on the front side.
Accordingly, by virtue of the variation in the size of the mouth openings on the front side, when viewing the front side in back-lighting, for example in daylight, it is possible to see an image with gray scales.
This image may involve digits, letters, symbols or representations for example of a face.
The differing diameters of the mouth openings on the front side are implemented by the duration for which the substrate is subjected to the action of laser light being varied in dependence on the desired diameter of the mouth opening.
Although a laser beam is very well focused, ultimately, as it increasingly burns into a substrate from the rear side, it produces a hole which is conical at least in the deepest region, due to the Gaussian distribution of the energy within the cross-section of the laser beam and at least as long as the duration of action of the laser beam is not still substantially prolonged after piercing the substrate. That conicity can be increased or attenuated by suitable optical systems arranged upstream of the point of impingement, for shaping the laser beam.
For, after piercing the substrate, the burnt passage is expanded to the full thickness of the laser beam and it is only after this has been achieved that the mouth opening on the exit side is approximately of the same size as the mouth opening on the entry side.
As this is precisely not wanted for influencing the size of the mouth openings on the exit side, that is to say the front side of the document, the through openings through the substrate are slightly conical when considered in longitudinal section.
In addition, due to the generation of heat by the laser light, the edges of the mouth openings, in particular on the exit side of the laser light, are rounded, at least insofar as the materials involved are relatively soft such as plastic materials.
By virtue of the conical shape of the holes which constitute the perforation pattern, the intended image, in contrast to the assertion in the above-mentioned patent application, can be recognized when considered in the back-lighting mode, irrespective of the side from which the substrate is viewed.
This is due to the fact that, even when viewing from the rear side, at which in fact the mouth openings are arranged regularly or irregularly in mutually juxtaposed relationship and are of the same diameter, the amount of the light passing thereto is nonetheless determined by the size of the mouth openings on the opposite side, the front side.
Accordingly therefore the intended image effect can be recognized when viewing from both sides. Such a security feature however is relatively easy to circumvent: thus for example a corresponding perforation pattern could be produced by means of a kind of sewing machine, for example by using two needles of differing thicknesses, or also by means of one and the same needle which is of a conical configuration at least in the front region, and simultaneous control of the depth of engagement of the needle.